When you create your project in Unity, for example, your first step is to name the project and select a location to save it, usually in a folder named after the game 'MyAwesomeGame' - then all your files are in MyAwesomeGame/. You would want to upload that entire directory to your destination of choice, like dropbox, or if you are using GitHub you can push all your code to a repo (if you don't know git, maybe avoid this route for now as it may be a lot to try to learn in addition to everything else). Whatever you choose, make it publicly available and share the link with your submission.
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Actually you want to filter out some of the folders inside "MyAwesomeGame", especially the `Library` folder, but also "Logs", "obj", and "Temp" (if there is one).
If you go with Git (highly recommended!), there are other hosting sites as well even if Github is most known. (GDTV uses Gitlab for example, and there is bitbucket.org, too)
Make sure you add a `.gitignore` file to your project, and add it *first*. A well-tuned ignore file will have the Library and other things listed so they won't get added into your git repository.